Arc heater apparatus employing fluid-cooled electrodes having permanent magnets to drive the arc therefrom

ABSTRACT

An arc heater having means forming an enclosed arc chamber, said means including first and second spaced electrodes electrically insulated from each other, the second electrode being the downstream electrode, the first and second electrodes being adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, means for admitting gas to be heated into the chamber and exhausting heated gas therefrom. Each of the first and second electrodes including a tip forming an arcing surface, each electrode tip having at least one space therein extending around the entire periphery of the tip, at least a portion of the space forming a fluid passageway for the flow of cooling fluid to conduct heat flux from the arcing surface, at least one permanent magnet mounted in the tip and occupying at least some of the remainder of the space in the tip, each electrode including supporting means for the tip secured thereto and including means for conducting fluid to and from the passageway in the tip, the permanent magnets within the tips creating a magnetic field with lines which are transverse to the arcing surfaces of both electrodes and which exert a force on the arc which causes the arc to move substantially continuously around and between the arcing surfaces of the tips.

United States Patent De Corso et al. A

[ 1 Sept. 5, 1972 ARC HEATER APPARATUS EMPLOYING FLUID-COOLED ELECTRODES HAVING PERMANENT Primary Examiner-Velodymyr Y. Mayewsky Attorney--A. T. Stratton MAGNETS TO DRIVE THE ARC THEREFROM [57] ABSTRACT Inventors! sel'flfillfl M- De Cor James M, An arc heater having means forming an enclosed arc W ll both f Pittsburgh, p chamber, said means including first and second spaced v electrodes electrically insulated from each other, the [73] Asslgnee' WW? Electnc Corporatlon second electrode being the downstream electrode, the

I P first and second electrodes being adapted to be con- [22] Filed: April 29, 1971 nected to termgzials of opposite polarity of a source of potential to pr uce and sustain an arc therebetween, [21] Appl' 138830 means for admitting gas to be heated into the chamber Related s Appfieafion Data and exhausting heated gas therefrom. Each of the first and second electrodes including a tip forming an arc- [62] DlVlSlOll of Ser. No. 4,488, Jan. 21, 1970, Pat. ing Surface, each electrode tip having at least one 3,610,796 space therein extending around the entire periphery of V A the tip, at least a portion of the space forming a fluid [52] US. Cl. ..219/383, 13/18, 219/123, passageway for the flow of cooling fluid to conduct 51 I t cl 313/ heat flux from the arcing surface, at least one perk 121 1/23. manent magnet mounted in the tip and occupying at 0 c least some of the remainder of the space in the tip, 13/18 313/153 2 20 each electrode including supporting means for the tip secured thereto and including means for conducting [56] References Clted fluid to and from the passageway in the tip, the per- UNITED STATES PATENTS manent magnets within the tips creating a magnetic field with lines which are transverse to the arcing sur- 2,286,211 6/1942 Dawson et al ..l3/l8 UX faces f both electrodes and wh exert a force on 3,369,067 2/1968 De COISO ..l3/l8 the are which eauses the are to move substantially 9 i f continuously around and between the arcing surfaces am er e f the ti 3,629,553 12/1971 Fey et al. ....219/383 I p 3,610,796 10/1971 De Corso et a1 ..l3/18 15 Claims, 6 Drawing Figures 1Q "2 n3 1 V uo 4 4 8 s N Q4 v I27 5 I05 I06 I04 Y M I2 "8 4 "6 '25 '08 p "7 I 4 -J I N N I r 7; x 1 Cl; I r/ PATENTEDSEP 5 I972 SHEET 1 OF 2 ARC HEATER APPARATUS EMPLOYING FLUID- COOLED ELECTRODES HAVING PERMANENT MAGNETS TO DRIVE THE ARC THEREFROM CROSS-REFERENCE TO RELATED APPLICATIONS 3 407,3 32, filed Oct. 29, 1964, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to electrodes employing magnetic fields transverse to an arc path from the tip thereof for setting up a force on the arc to cause the arc to move substantially continuously around the arcing surface of the electrode.

2. Description of the Prior Art It has been suggested heretofore that a single permanent magnet located near an arcing surface but not within the tip, with its north and south poles axially spaced, be used to move an arc; such a suggestion was made in application Ser. No. 407,332, filed Oct. 29, 1964.

A number of patents have issued on prior art electrodes in which a magnetic field coil is located in a tip generally annular in shape and generally U-shaped in cross-section, with a fluid passageway U-shaped in cross section extending around the entire tip to conduct cooling fluid near the arcing surface and remove heat flux therefrom. Such prior art electrodes with field coils are exemplified by U.S. Pat. No. 3,369,068 to P. F. Kienast issued Feb. 13, 1968, and U.S. Pat. No. 3,398,229 to Decorso et al issued Aug. 20, 1968.

Such prior art electrodes employing field coils for setting up magnetic fields to rotate the are are complicated by the necessity of providing electrical insulation between the magnetic field coil and the electrode structure, and require that leads for energizing the field coil pass through at least a portion of the electrode structure and usually pass the entire long distance between the electrode tip and the upper or head portion of the electrode. Furthermore, magnetic field coils are relatively expensive compared to permanent magnets.

Additionally, magnetic field coils almost always have the north and south poles of the coil disposed with respect to each other in a direction parallel to the longitudinal axis of the electrode, with the result that many flux lines leave the electrode tip in a direction which is substantially perpendicular to the arcing surface rather than transverse to the arc path; the magnetic field over the arcing surface is non-uniform and there is a tendency for the arc to be driven toward the inside annular surface of the electrode tip as a result of the configuration of the magnetic field lines, or more precisely, the shape of the magnetic field, density, and strength of field components in certain directions.

SUMMARY OF THE INVENTION Our electrode employing apermanent magnet is cheaper than prior art structures, needs no electrical insulation or electrical connections to a field coil, and furthermore in our electrode employing a permanent magnet with the north and south poles of the magnet being selectively the outside annular surface and the inside annular surface, or vice versa, we produce a magnetic field which extends transversely to (substantially radially across) a much larger portion of the arcing surface of the electrode tip. This is also true of other embodiments of for invention. An arc heater having electrodes embodying our invention is very simple and easy to construct compared to prior art are heaters.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical section partially broken away through an electrode and electrode tip according to one embodiment of our invention;

FIG. 2 is a section through the lines Il-II of FIG. 1;

FIG. 3 is an electrode tip partially broken away according to a second embodiment of our invention in which two radially spaced annular permanent magnets are employed;

FIG. 4 is an additional embodiment of our invention employing two radially spaced annular permanent magnets;

FIG. 5 is a cross-sectional view through an arc heater employing electrodes according to our invention in which annular or ring-shaped permanent magnets are mounted in the tips of the electrodes; and

FIG. 6 shows schematically segmented permanent magnet field producing means including peripherally spaced radially extending bars all having the same magnetic pole at the inner ends thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 the electrode generally designated 11 is seen to include a supporting column portion generally designated 12 and an electrode tip generally designated 13. The supporting column is shown as consisting'of two coaxially mounted tubes 14 and 15, the tubes being radially spaced from each other to provide a cylindrical fluid flow passageway 16 therebetween. The outer tube 14 has a flaring transverse flange portion 17 at the lower end thereof with threads 18 to receive in threaded engagement the electrode tip generally designated 13. An O-ring 20 disposed within a suitable annular groove provides a fluid tight seal.

The electrode tip generally designated 13 includes an annular shell composed of material having high thermal and electrical conductivity with an outer wall portion 24 of larger diameter and an inner wall portion 25 of smaller diameter, and an annular bottom portion 26, shown as curved with the inner and outer edges being extensions of wall portions 25 and 24. Lead 28 symbolizes means for connecting the electrode to one terminal of a source of potential, the other terminal of opposite polarity being connected to a melt 30 which is at least partially conductive, cylinder 14 being conductive for bringing current to the tip to produce the arc 33 from the tip to the melt. An axial portion of the outside surface of outer wall 24 is coated with a ceramic coating 35 to provide thermal insulation.

The aforementioned tubular member of the supporting column provides inside thereof a fluid passageway 37 and is seen to have at the lower end thereof a thickened flaring portion 40 of considerably enlarged outside diameter which has the outside wall 42 thereof spaced from the inside surface of outer wall 24 of the electrode tip to form a cylindrical passageway 44 which extends around the entire tip, which passageway 44 communicates at one end thereof with the aforementioned cylindrical passageway 16 between the tubular support members 14 and 15, and communicates at the other end thereof with passageway 37.

The lower end portion of enlarged diameter 40 is terminated at a predetermined axial position with respect to the electrode and tip and has secured to the lower surface 46 thereof by any convenient means, not shown for convenience of illustration, a permanent magnet generally designated 48. Member 15 including portion 40 may be made of iron and magnet 48 secured thereto by magnetic attraction. On the other hand, where considerations of magnetic field strength at the arcing surface make a low reluctance path within the tip which may be followed by some of the lines of force undesirable, the sleeve and collar portion may be made of nonmagnetic material and the magnet secured thereto by small peripherally spaced screws inset so as not to obstructthe flow of fluid. The permanent magnet 48 is ring-shaped with an outer annular wall surface 51 and an inner annular wall surface 52, these being preferably substantially flat, both these surfaces being spaced from the adjacent inside wall surfaces of the tip, the bottom of the magnet 48 also being spaced from the adjacent inner surface of the electrode tip so as not to obstruct the aforementioned fluid flow passageway 44 which extends around the permanent magnet on three sides thereof and around the entire tip, passageway 44 as aforementioned communicating at the inside annular opening thereof with the passageway 37 in cylinder 15, forming a complete fluid flow passageway for conducting cooling fluid near the arcing surface 29 in a path U-shaped in cross section which extends annularly around the entire tip, the coolant entering one and thereafter being removed through the other fluid flow passageway in the supporting structure generally designated 12.

It will be understood that tube 15 is illustrated as supported from above and held in position by means, not shown for convenience of illustration, to thereby maintain the permanent magnet 48 in desired position within the tip and spaced therefrom, although other long-known and conventional spacing means could be employed, such for example as spaced studs so positioned as not to substantially impede the flow of cooling fluid.

Permanent magnet 48 by way of illustration has the outside substantially flat annular surface 51 thereof forming the north pole of the magnetic structure and the inside substantially flat annular wall surface 52 thereof forming the south magnetic pole. Lines of force of similar polarity leaving or entering the inside wall surface 52 of the magnet are repelled from each other because of their like polarity, are bent around the arcing surface of the electrode, and enter the magnet again at the surface of opposite polarity 51, these lines of force being illustrated at 57 and 58. It will be understood that the north and south poles may be reversed if desired.

The permanent magnet may be composed of ferrite material or ceramic material and by suitable choice of material and dimensions may supply a magnetic field having a strength or flux density, when the field emerges from the magnetic pole surfaces, of several thousand gauss, which is ample to cause rotation of the are at a speed which prevents substantial erosion of material from the arcing surface, the force exerted on the arc and the speed of rotation thereof being a function of the product of the magnetic field strength and the arc current. 7

Particular reference is made to FIG. 2, a cross section along the line lI-II of FIG. 1. The annular passageway 44 is seen both on the outside and inside of the permanent magnet 48. The hollow central depression in the electrode tip resulting from its annular ring configuration is seen at 61.

Particular reference is made now to FIG. 3 which shows an electrode according to a second embodiment of our invention, only one half of the tip being shown as needed to fully illustrate this embodiment of the invention. In FIG. 3 the shell of the tip is shown at 65 having an annular substantially flat bottom portion forming an arcing surface with a recessed control closure portion 85, and disposed within the shell are radially spaced permanent magnets 71 and 72 both being in the form of rings with their upper ends oppositely poled as shown, the magnets being separated by an annular spacer member 73 composed of any suitable diamagnetic material such for example as epoxy resin. An annular ring composed of iron or other ferromagnetic material is shown at 75 closing the flux path internal to the electrode. Members 71, 72, 73 and 75 are spaced from the adjoining inside walls of the shell 65 to provide a fluid passageway 77 around the entire tip for the flow of cooling fluid to conduct heat flux from the arcing surface. Any suitable means, not shown for convenience of illustration, may be employed for holding the two annular permanent magnets, the spacer 73, and the iron ring member 75 in position within the electrode tip. The fluid in passageway 77 which communicates with passageway 16, not shown, passes through the cylindrical space 79 within magnet ring 72, thence through the central passageway 80 of the iron ring 75 and into a passageway, not shown, corresponding to passageway 37, FIG. 1. As aforementioned, the upper opening at the outside portion of passageway 77 it is understood communicates with a fluid channeling passageway, not shown for convenience of illustration, between'coaxially aligned cylinders and corresponding to passageway 16, FIG. 1.

The magnetic field illustrated at 81 extends between the lower south pole of outer ring magnet 71 and the lower north pole of inner ring magnet 72, extends across or transverse to the arcing surface and transverse to the arc path and exerts a force on the are according to the left-hand rule or Flemings rule which causes the arc to rotate in an annular path around the arcing surface. A ceramic heat shield 83 covers a portion of the outside wall of shell 65 which will not be used as part of the arcing surface. In FIG. 3 in addition to the ceramic coating or other heat shield material 83, the central hub portion 85 of the shell 65 is seen to be covered by coating 86 of ceramic or other refractory since this portion 85 may not be as well cooled by the circulating fluid as are the portions of the electrode tip and shell adjacent the U-shaped passageway 77.

The relating polarities of magnets 71 and 72 may be reversed, if desired.

Particular reference is made now to FIG. 4. The shell 88 forming the tip may be substantially cylindrical in shape and the magnets 71 and 72 are separated by the spacer 73' of diamagnetic material and the iron ring 75' provides a closed flux path within the electrode. It is understood that members 71, 72, 73' and 75 are supported and maintained in position by any suitable means, not shown for convenience of illustration. A ceramic coating 83' extends along the entire length of the outside wall of shell 88 and extends a predetermined distance toward the axial center of the tip as shown. A disc-shaped coating 91 of ceramic material is also provided on the under surface of the tip, the outer edge of the disc shaped portion 91 being spaced from the inwardly extending edge of ceramic coating 83 to provide an exposed arcing surface 93 of predetermined width from which the arc 94 takes place. The magnetic field is shown at 95; it is seen to extend between member 71' and 73' transversely across the arcing surface 93 and exerts a force on the are which causes the arc 94 to rotate in a substantially annular path around the arcing surface.

Particular reference is made to FIG. 5 in which an arc heater is shown employing two axially spaced electrodes, both being similar to the electrode of FIG. 1 and generally designated 101 and 102 respectively. Electrode 101 has a tip 104, a permanent magnet 105, and a fluid passageway 106. Electrode 101 is held in position within the pressure vessel 108 by an annular ring and supporting member 110 electrically insulated from the electrode by an insulating sleeve 111. The ring support and spacing member 110 has a plurality of peripherally spaced bores or passageways extending axially therethrough, two of these being shown at 1 12 and 113, for admitting gas to be heated into the arc chamber 114 between the electrodes.

The aforementioned second electrode 102, which is the downstream electrode, has a tip 116, a permanent magnet 1 17, and a passageway 118 for the flow of cooling fluid. Leads 121 and 122 connect the electrodes 101 and 102 to terminals of opposite polarity of a source of potential to produce and sustain the arc 123 between electrodes. The aforementioned downstream electrode 102 is mounted and held in position within the pressure vessel 108 by an annular ring member 124 which may be composed of insulating material or may be composed of metal in which case a sleeve 125 composed of electrically insulating material is interposed between the electrode and the support member 124.

The construction of the downstream electrode dif fers slightly from that of the upstream electrode. The central opening formed by the annular ring configuration of the tip 104 of electrode 101 is seen to be closed at 127, whereas the inside wall of the smaller diameter of the tip or shell 116 of electrode 102 forms a cylindrical space in which is fixedly secured a generally cylindrical nozzle member 129 having an exhaust vent 130 communicating between the arc chamber 114 and the outside of the pressure vessel, and through which gas heated by the are 123 exits from the arc heater.

In the operation of the apparatus of FIG. 5 the two ring-shaped permanent magnets may be poled as shown, magnet setting up a field which is transverse to the arcing surface of tip 104 and magnet 1 17 of electrode 102 setting up a magnetic field which is transverse to the arcing surface of tip 116. It is seen that the inside wall surface of smaller diameter of magnet 105 has the same magnetic polarity as the inside wall surface of smaller diameter of magnet 117; the field is set up at the two electrodes tend to oppose each other and enhance the strength of the transverse component of the field which lies across each arcing surface. Both magnets exert a force on the are 123 which cause the arc to rotate in an annular path between electrodes. It is to be noted that the forces exerted on the arc 123 by the two magnets are such as to add and cause the are 123 to rotate in the same angular direction between electrodes.

In accordance with long-established practice, the

permanent magnets may be solid or laminated solid magnets being shown for ease of illustration.

Particular reference is made to FIG. 6. Discrete radially extending peripherally spaced magnetic bars 150, which may extend perpendicular to the axis of the electrode, have all their inner ends of like polarity and all their outer ends of like polarity. A magnetic field transverse to the arcing surface and similar to fields 57 and 58, FIG. 1, is set up. Preferably the bars extend at least the major portion of the distance between the wall of smaller diameter of the tip and the wall of larger diameter of the tip.

In the arc heater of FIG. 5, the permanent magnet configurations and tip configurations of FIGS. 3 and 4 may be substituted for those shown.

The foregoing written description and the drawings are illustrative only and are not to be interpreted in a limiting sense.

We claim as our invention:

1. An arc heater comprising in combination, means forming an enclosed arc chamber, said means including first and second spaced electrodes electrically insulated from each other and from the remainder of the chamber forming means, the second electrode being the downstream electrode, the first and second electrodes being adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, means for admitting gas to be heated into the arc chamber and exhausting heated gas therefrom, each of the first and second electrodes including an annular at least partially hollow tip having permanent magnet means mounted therein and spaced from the adjacent inside wall surfaces of the tip to form a fluid flow passageway, each of the permanent magnet means producing a magnetic field transverse to the arcing surface of the adjacent tip with lines of force extending in a radial direction across the tip from the axis of the electrode, said magnetic fields exerting a force on the are which causes the arc to move substantially continuously in an annular path around and between the electrodes.

2. An arc heater according to claim 1 in which the permanent magnet means in each electrode includes a single ring-shaped permanent magnet, the inside wall of smaller diameter of the ring-shaped permanent magnet forming one magnetic pole surface and the outside wall of larger diameter of the permanent magnet forming the other magnetic pole surface, the permanent magnet generating a field which is transverse to the arcing surface and has lines of force which extend in a radial direction across said arcing surface.

3. An arc heater according to claim 2 in which the exhaust means has a nozzle member having a vent extending axially therethrough, said nozzle member being centrally disposed in the downstream electrode.

4. An arc heater according to claim 1 in which each of the electrodes includes a pair of coaxially mounted radially spaced tubes forming a passageway therebetween communicating with the passageway in the tip for bringing fluid to the passageway in the tip.

5. An arc heater according to claim 1 in which the means for bringing gas into the arc chamber to be heated by the arc therein includes a plurality of peripherally spaced passageways extending through the means forming an enclosed arc chamber.

6. In an arc heater of the type having means forming an enclosed arc chamber with means for admitting gas to be heated into the chamber and means for exhausting gas from the chamber, and a pair of spaced fluid cooled electrodes in the chamber adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, each electrode having a fluid cooled tip generally annular in shape and generally U-shaped in cross section with an outer generally annular wall of larger diameter and an inner generally annular wall of smaller diameter, with magnetic field producing means in the tip of each electrode forproducing a field which exerts a force on the are between electrodes and causes said are to move substantially continuously around and between the tips, the improvement which comprises permanent magnet means in each tip spaced from adjacent inside wall surfaces of the tip to provide a passageway for cooling fluid, the permanent magnet means being constructed and arranged to form one magnetic pole adjacent the inside surface of the wall of small diameter of the tip around the entire periphery thereof and the other magnetic pole adjacent the inside surface of the wall of larger diameter of the tip around the entire periphery thereof, the lines of force of the magnetic field extending generally radially from the axis of the electrode and transverse to the arcing surface of the tip.

7. An arc heater according to claim 6 in which the permanent magnet means in each electrode includes a single ring-shaped permanent magnet having an inside wall of smaller diameter and an outside wall of relatively larger diameter, the inside wall of the magnet forming one magnetic pole around the entire periphery thereof and the outside wall of the magnet forming the opposite magnetic pole around the entire periphery thereof.

8. An arc heater according to claim 6 in which the permanent magnet means in each electrode includes a plurality of peripherally spaced radially extending bar magnets all extending at least the major portion of the distance between the wall of smaller diameter of the tip and wall of larger diameter of the tip, the inner ends of all of the bar magnets having the same magnetic polarity and the outer ends of all of the bar magnets having the same magnetic polarity.

9. In an arc heater of the type having means forming or enclosed arc chamber with means for admitting gas to be heated into the chamber and means for exhausting gas from the chamber, and a pair of spaced fluidcooled electrodes in the chamber adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, each electrode having a fluid cooled tip generally annular in shape and generally U-shaped in cross section with an outer generally annular wall of larger diameter and an inner generally annular wall of smaller diameter, with magnetic field producing means in the tip for producing a field which exerts a force on the are between electrodes and causes said arc to move substantially continuously around the tips, the improvement which comprises permanent magnet means in each tip spaced from adjacent inside wall surfaces of the tip to provide a passageway for cooling fluid, the permanent magnet means including two coaxially aligned radially spaced ring magnets, each of said ring magnets having axially spaced magnetic poles at the upper and lower surfaces thereof, the ring magnets being oppositely poled with respect to each other.

10. An arc heater according to claim 9 including spacer means composed of diamagnetic material between the two ring magnets.

11. An arc heater comprising in combination, means forming an enclosed arc chamber, said means including first and second spaced electrodes electrically insulated from each other, the second electrode being the downstream electrode, the first and second electrodes being adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, means for admitting gas to be heated into the chamber and exhausting heated gas therefrom, each of the first and second electrodes including a tip forming an arcing surface, each electrode tip having at least one space therein extending around the entire periphery of the tip, at least a portion of the space forming a fluid passageway for the flow of cooling fluid to conduct heat flux from the arcing surface,

at least one permanent magnet mounted in the tip and occupying at least some of the remainder of the space in the tip, each electrode including supporting means for the tip secured thereto and including means for conducting fluid to and from the passageway in the tip, the permanent magnets within the tips creating a magnetic field with lines which are transverse to the arcing surfaces of both electrodes and which exert a force on the are which causes the arc to move substantially continuously around and between the arcing surfaces of the tips.

12. Are heater apparatus according to claim 11 in which the permanent magnet in each electrode tip is ring-shaped with a substantially flat inner wall surface of smaller diameter and a substantially flat outer wall surface of larger diameter, the wall surfaces of each magnet being substantially parallel to the axis of the electrode, the inner wall surface and the outer wall surface of each permanent magnet forming opposite magnetic poles of the permanent magnet.

13. Arc heater apparatus according to claim 12 in which the outer wall surface of the permanent magnet in one tip has the same magnetic polarity as the outer wall surface of the permanent magnet in the other tip.

14. Arc heater apparatus according to claim 11 in which each electrode tip includes two radially spaced substantially axially aligned permanent magnets mounted within the space within the electrode tip, the axial end surfaces of the permanent magnets forming the magnetic poles thereof, in each tip the poles of one magnet being oppositely disposed with respect to the corresponding poles of the other magnet. 

1. An arc heater comprising in combination, means forming an enclosed arc chamber, said means including first and second spaced electrodes electrically insulated from each other and from the remainder of the chamber forming means, the second electrode being the downstream electrode, the first and second electrodes being adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, means for admitting gas to be heated into the arc chamber and exhausting heated gas therefrom, each of the first and second electrodes including an annular at least partially hollow tip having permanent magnet means mounted therein and spaced from the adjacent inside wall surfaces of the tip to form a fluid flow passageway, each of the permanent magnet means producing a magnetic field transverse to the arcing surface of the adjacent tip with lines of force extending in a radial direction across the tip from the axis of the electrode, said magnetic fields exerting a force on the arc which causes the arc to move substantially continuously in an annular path around and between the electrodes.
 2. An arc heater according to claim 1 in which the permanent magnet means in each electrode includes a single ring-shaped permanent magnet, the inside wall of smaller diameter of the ring-shaped permanent magnet forming one magnetic pole surface and the outside wall of larger diameter of the permanent magnet forming the other magnetic pole surface, the permanent magnet generating a field which is transverse to the arcing surface and has lines of force which extend in a radial direction across said arcing surface.
 3. An arc heater according to claim 2 in which the exhaust means has a nozzle member having a vent extending axially therethrough, said nozzle member being centrally disposed in the downstream electrode.
 4. An arc heater according to claim 1 in which each of the electrodes includes a pair of coaxially mounted radially spaced tubes forming a passageway therebetween communicating with the passageway in the tip for bringing fluid to the passageway in the tip.
 5. An arc heater according to claim 1 in which the means for bringing gas into the arc chamber to be heated by the arc therein includes a plurality of peripherally spaced passageways extending through the means forming an enclosed arc chamber.
 6. In an arc heater of the type having means forming an enclosed arc chamber with means for admitting gas to be heated into the chamber and means for exhausting gas from the chamber, and a pair of spaced fluid cooled electrodes in the chamber adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, each electrode having a fluid cooled tip generally annular in shape and generally U-shaped in cross section with an outer generally annular wall of larger diameter and an inner generally annular wall of smaller diameter, with magnetic field producing means in the tip of each electrode for producing a field which exerts a force on the arc between electrodes and causes said arc to move substantially continuously around and between the tips, the improvement which comprises permanent magnet means in each tip spaced from adjacent inside wall surfaces of the tip to provide a passageway for cooling fluid, the permanent magnet means being construcTed and arranged to form one magnetic pole adjacent the inside surface of the wall of small diameter of the tip around the entire periphery thereof and the other magnetic pole adjacent the inside surface of the wall of larger diameter of the tip around the entire periphery thereof, the lines of force of the magnetic field extending generally radially from the axis of the electrode and transverse to the arcing surface of the tip.
 7. An arc heater according to claim 6 in which the permanent magnet means in each electrode includes a single ring-shaped permanent magnet having an inside wall of smaller diameter and an outside wall of relatively larger diameter, the inside wall of the magnet forming one magnetic pole around the entire periphery thereof and the outside wall of the magnet forming the opposite magnetic pole around the entire periphery thereof.
 8. An arc heater according to claim 6 in which the permanent magnet means in each electrode includes a plurality of peripherally spaced radially extending bar magnets all extending at least the major portion of the distance between the wall of smaller diameter of the tip and wall of larger diameter of the tip, the inner ends of all of the bar magnets having the same magnetic polarity and the outer ends of all of the bar magnets having the same magnetic polarity.
 9. In an arc heater of the type having means forming or enclosed arc chamber with means for admitting gas to be heated into the chamber and means for exhausting gas from the chamber, and a pair of spaced fluid-cooled electrodes in the chamber adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, each electrode having a fluid cooled tip generally annular in shape and generally U-shaped in cross section with an outer generally annular wall of larger diameter and an inner generally annular wall of smaller diameter, with magnetic field producing means in the tip for producing a field which exerts a force on the arc between electrodes and causes said arc to move substantially continuously around the tips, the improvement which comprises permanent magnet means in each tip spaced from adjacent inside wall surfaces of the tip to provide a passageway for cooling fluid, the permanent magnet means including two coaxially aligned radially spaced ring magnets, each of said ring magnets having axially spaced magnetic poles at the upper and lower surfaces thereof, the ring magnets being oppositely poled with respect to each other.
 10. An arc heater according to claim 9 including spacer means composed of diamagnetic material between the two ring magnets.
 11. An arc heater comprising in combination, means forming an enclosed arc chamber, said means including first and second spaced electrodes electrically insulated from each other, the second electrode being the downstream electrode, the first and second electrodes being adapted to be connected to terminals of opposite polarity of a source of potential to produce and sustain an arc therebetween, means for admitting gas to be heated into the chamber and exhausting heated gas therefrom, each of the first and second electrodes including a tip forming an arcing surface, each electrode tip having at least one space therein extending around the entire periphery of the tip, at least a portion of the space forming a fluid passageway for the flow of cooling fluid to conduct heat flux from the arcing surface, at least one permanent magnet mounted in the tip and occupying at least some of the remainder of the space in the tip, each electrode including supporting means for the tip secured thereto and including means for conducting fluid to and from the passageway in the tip, the permanent magnets within the tips creating a magnetic field with lines which are transverse to the arcing surfaces of both electrodes and which exert a force on the arc which causes the arc to move substantially continuously around and between the arcing surfaces of the tips.
 12. ArC heater apparatus according to claim 11 in which the permanent magnet in each electrode tip is ring-shaped with a substantially flat inner wall surface of smaller diameter and a substantially flat outer wall surface of larger diameter, the wall surfaces of each magnet being substantially parallel to the axis of the electrode, the inner wall surface and the outer wall surface of each permanent magnet forming opposite magnetic poles of the permanent magnet.
 13. Arc heater apparatus according to claim 12 in which the outer wall surface of the permanent magnet in one tip has the same magnetic polarity as the outer wall surface of the permanent magnet in the other tip.
 14. Arc heater apparatus according to claim 11 in which each electrode tip includes two radially spaced substantially axially aligned permanent magnets mounted within the space within the electrode tip, the axial end surfaces of the permanent magnets forming the magnetic poles thereof, in each tip the poles of one magnet being oppositely disposed with respect to the corresponding poles of the other magnet.
 15. Arc heater apparatus according to claim 11 in which the permanent magnet of each electrode tip consists of a plurality of radially extending bar magnets disposed around entire periphery of the tip at peripherally spaced intervals, all of the inner ends of the bar magnets of each tip being of like magnetic polarity. 